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/**
* This file is part of Haketilo.
*
* Function: Implement a lock (aka binary semaphore aka mutex).
*
* Copyright (C) 2021 Wojtek Kosior
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* As additional permission under GNU GPL version 3 section 7, you
* may distribute forms of that code without the copy of the GNU
* GPL normally required by section 4, provided you include this
* license notice and, in case of non-source distribution, a URL
* through which recipients can access the Corresponding Source.
* If you modify file(s) with this exception, you may extend this
* exception to your version of the file(s), but you are not
* obligated to do so. If you do not wish to do so, delete this
* exception statement from your version.
*
* As a special exception to the GPL, any HTML file which merely
* makes function calls to this code, and for that purpose
* includes it by reference shall be deemed a separate work for
* copyright law purposes. If you modify this code, you may extend
* this exception to your version of the code, but you are not
* obligated to do so. If you do not wish to do so, delete this
* exception statement from your version.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <https://www.gnu.org/licenses/>.
*
* I, Wojtek Kosior, thereby promise not to sue for violation of this file's
* license. Although I request that you do not make use this code in a
* proprietary program, I am not going to enforce this in court.
*/
/*
* Javascript runs single-threaded, with an event loop. Because of that,
* explicit synchronization is often not needed. An exception is when we use
* an API function that must wait. Ajax is an example. Callback passed to ajax
* call doesn't get called immediately, but after some time. In the meantime
* some other piece of code might get to execute and modify some variables.
* Access to WebExtension local storage is another situation where this problem
* can occur.
*
* This is a solution. A lock object, that can be used to delay execution of
* some code until other code finishes its critical work. Locking is wrapped
* in a promise.
*/
function make_lock() {
return {free: true, queue: []};
}
function _lock(lock, cb) {
if (lock.free) {
lock.free = false;
setTimeout(cb);
} else {
lock.queue.push(cb);
}
}
function lock(lock) {
return new Promise((resolve, reject) => _lock(lock, resolve));
}
function unlock(lock) {
if (lock.free)
throw new Exception("Attempting to release a free lock");
if (lock.queue.length === 0) {
lock.free = true;
} else {
let cb = lock.queue[0];
lock.queue.splice(0, 1);
setTimeout(cb);
}
}
/*
* EXPORTS_START
* EXPORT make_lock
* EXPORT lock
* EXPORT unlock
* EXPORTS_END
*/
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